In recent years, to protect the environment and prevent global warming, demand has been rising for keeping down the consumption of fossil fuels. This demand has had an impact on various manufacturing industries. For example, even for automobiles, which are essential means of transport for daily life and activities, are no exceptions. Improvement of fuel economy etc. by lightening of the weight of the chasses are being sought. However, in automobiles, just realizing lighter weight of a chassis is not allowed in terms of product performance. It is necessary to secure suitable safety.
Much of the structure of an automobile is formed by an iron-based material, in particular steel sheet. Reduction of the weight of this steel sheet is important in lightening the weight of the chassis. However, as explained above, just reducing the weight of the steel sheet is not allowed. Securing mechanical strength of the steel sheet is simultaneously sought. Similar demands are made on steel sheet in various other manufacturing industries in addition to the automobile manufacturing industry. Accordingly, steel sheet which is raised in mechanical strength so as to enable the thickness to be reduced compared with the conventionally used steel sheet while maintaining or improving the mechanical strength is being researched and developed.
In general, a material which has a high mechanical strength tends to fall in shape freezability after bending or other shaping and is difficult to form into a complicated shape. As one means for solving this problem with shapeability, the so-called “hot press method (also called the hot stamp method, hot pressing method, or the die quench method)” may be mentioned. With this hot press method, the material to be shaped is heated once to a high temperature to soften the steel sheet by heating, then the steel sheet is press formed to shape it, then is cooled. According to this hot press method, the material is heated once to a high temperature to make it soften, so the material can be easily press formed. Furthermore, due to the hardening effect caused by the cooling after shaping, the material can be raised in mechanical strength. Therefore, the hot press method enables a shaped product to be obtained which achieves both good shape freezability and high mechanical strength.
However, if applying this hot press method to steel sheet, heating the steel sheet to an 800° C. or more high temperature causes the surface of the steel sheet to oxidize and scale (oxides) to form. Therefore, after performing hot press forming, a step of removing this scale (descaling step) becomes necessary and the productivity falls. Further, in members which require corrosion resistance etc., the surfaces of the members have to be treated to make them rustproof or covered by metal after being worked. A surface cleaning step and surface treatment step become necessary, so the productivity further falls.
As a method for suppressing such a drop in productivity, the method of providing the steel sheet with a covering may be mentioned. As the covering of the steel sheet, in general an organic material or inorganic material or other various materials are used. Among these, galvannealed steel sheet, which has a sacrificial corrosion action against the steel sheet, is being widely used for automobile steel sheet etc. from the viewpoint of the anticorrosion performance and steel sheet production technology. However, the heating temperature (700 to 1000° C.) in hot press forming is higher than the decomposition temperature of the organic material or the melting point and boiling point of the Zn or other metal. When using a hot press for heating, the surface coating and plating layer evaporate causing remarkable deterioration of the surface properties.
Therefore, as the steel sheet to which the hot press method which is accompanied with high temperature heating is applied, it is desirable to use steel sheet which is provided with an Al-based metal covering, which has a higher boiling point than an organic material covering or a Zn-based metal covering, or an Al plated steel sheet. Here, an “Al plated steel sheet” includes sheets to which elements other than Al have been added to improve the characteristics of the plating layer. The Al of the plating layer should be, by mass %, 50% or more.
By providing the Al-based metal covering, it is possible to prevent scale from forming on the surface of the steel sheet and therefore descaling and other steps become unnecessary, so the shaped product is improved in productivity. Further, an Al-based metal covering also has a rustproofing effect, so the corrosion resistance is also improved. The method of hot pressing steel sheet which comprises steel sheet which has a predetermined chemical composition and is provided with an Al-based metal covering is disclosed in PLT 1.
However, when providing an Al-based metal covering, depending on the conditions of the preheating before the hot press forming, the Al covering will melt, then Fe will diffuse from the steel sheet and cause the formation of an Al—Fe alloy layer and, further, growth of the Al—Fe alloy layer until the surface of the steel sheet becomes an Al—Fe alloy layer. This Al—Fe alloy layer is extremely hard, so there was the problem that contact with the die at the time of press forming caused work marks on the shaped product.
An Al—Fe alloy layer is lower in slip at its surface and is poorer in lubricity. Furthermore, this Al—Fe alloy layer is hard and easily fractures. The plating layer suffers from cracks and powdering etc., so the shapeability falls. Further, any peeled off Al—Fe alloy layer sticks to the die or the surface of Al—Fe alloy layer of the steel sheet, is strongly rubbed against, and sticks to the die or Al—Fe intermetallic compounds derived from the Al—Fe alloy layer to adhere to the die and cause the shaped product to decline in quality. For this reason, it is necessary to periodically remove the Al—Fe intermetallic compounds which have adhered to the die. This becomes one cause of a drop in productivity of the shaped product or an increase in the production costs.
Furthermore, an Al—Fe alloy layer is low in reactivity with the usual phosphate treatment. Therefore, the surface of the Al—Fe alloy layer cannot be formed with a chemically converted coating (phosphate coating) as pretreatment for electrodeposition painting. Even when a chemically converted coating is not formed, if making the material good in coating adhesion then making the amount of deposition of Al sufficient, the coated corrosion resistance also will become excellent, but if increasing the amount of deposition of Al, adhesion of Al—Fe intermetallic compounds to the die will increase.
Adhesion of Al—Fe intermetallic compounds include the case where peeled off parts of the Al—Fe alloy layer deposit and the case where the Al—Fe alloy layer surface is strongly rubbed against and deposits. When hot press forming steel sheet which has a surface coating, if improving the lubricity, the strong rubbing and adhesion by the surface of the Al—Fe alloy layer are eased. However, improvement of the lubricity is not effective for alleviating the deposition of peeled off parts of the Al—Fe alloy layer on the die. To alleviate the deposition of peeled off parts of the Al—Fe alloy layer on the die, it is most effective to reduce the amount of deposition of Al on the Al plating. However, if reducing the amount of deposition of Al, the corrosion resistance deteriorates.
Therefore, steel sheet which prevents the shaped product from being formed with work marks is disclosed in PLT 2. The steel sheet which is disclosed in PLT 2 is steel sheet which has a predetermined chemical composition on the surface of which an Al-based metal covering is provided and, furthermore, on the surface of that Al-based metal covering an inorganic compound coating, organic compound coating, or composite compound coating of the same which contains at least one of Si, Zr, Ti, or P is formed. In the steel sheet which is formed with such a surface coating like that disclosed in PLT 2, even at the time of the press forming after heating, the surface coating will never peel off and therefore it is possible to prevent the formation of work marks at the time of press forming. However, with the surface coating which is described in PLT 2, a sufficient lubricity cannot be obtained at the time of press forming, so improvement etc. in the lubricant are sought.
PLT 3 discloses a method of solving the problem of surface deterioration of galvanized steel sheet due to evaporation of the galvanization layer in hot pressing of galvanized steel sheet. That is, it causes the formation of a high melting point zinc oxide (ZnO) layer as a barrier layer on the surface of the galvanization layer to thereby prevent the evaporation of Zn in the Zn plating layer at the bottom layer. However, the method which is disclosed in PLT 3 is predicated on the steel sheet having a galvanization layer. The Al content in the galvanization layer is allowed to be up to 0.4%. However, the content of Al is desirably small. The method which is disclosed in. PLT 3 is for preventing evaporation of Zn from the Zn plating layer. Al is included only incidentally. However, with incidental inclusion of Al in the Zn plating layer, it is not possible to completely prevent the evaporation of Zn in the Zn plating layer. Therefore, the general practice is to use Al plated steel sheet which has high boiling point Al as a main component.
PLT 4 discloses a method of applying a wurtzite type compound to the surface of an Al plated steel sheet. The method which is disclosed in PLT 4 improves the hot lubricity and the chemical convertability and secures adhesion of the surface coating before hot press forming by adding a binder ingredient to the surface coating. However, the binder of the method which is disclosed in PLT 4 ends up breaking down due to heat at the time of hot press forming and therefore there was the problem that the wurtzite type compound fell in coating adhesion from the steel sheet at the time of shaping.
PLT 5 discloses galvannealed steel sheet which is formed with a surface coating layer which contains Zn hydroxide and Zn sulfate. However, the steel sheet which is disclosed in PLT 5 forms a surface coating layer on the galvannealed steel sheet, so while it is excellent in corrosion resistance, there was the problem that the zinc in the galvannealed layer ended up evaporating at the time of hot pressing. Further, both surfaces of the steel sheet which is disclosed in PLT 5 are formed with an oxide layer which has 3Zn(OH)2.ZnSO4.nH2O (n=0 to 5). ZnSO4 dissolves the Al plating layer, so an Al plated steel sheet could not be used.
PLT 6 discloses steel sheet which is comprised of an Al plated steel sheet which is formed with a surface coating layer which contains a Zn compound which is selected from Zn sulfate, Zn nitrate, and Zn chloride. However, an aqueous solution of Zn sulfate, Zn nitrate, or Zn chloride is high in pH, so when coating the treatment solution when forming the surface coating layer, acts to dissolve the Al plated steel sheet. As a result, there was the problem that the coated corrosion resistance was degraded. Further, while the cause is not certain, there was the problem that the weldability was also degraded. This problem was particularly remarkable when including Zn sulfate and Zn nitrate as the Zn compound.
PLT 7 discloses steel sheet which is comprised of an Al plated steel sheet which is formed with a surface coating layer which contains a vanadium compound, a phosphoric acid compound, and at least one type of metal compound which is selected from Al, Mg, and Zn. However, the surface coating layer of the steel sheet which is disclosed in PLT 7 contains a vanadium compound, so the valence of the vanadium compound causes various colors to be formed and therefore there was the problem of an uneven appearance.